1. Field of the Invention
This invention relates to a method of preparing an electrostatically imaged printing plate, and to a method of printing using a plate prepared by such a method. More particularly, the method of this invention comprises imaging a substrate electrostatically with a toner composition, then heating the imaged substrate via non-contact (e.g. radiant heating) a first time to xe2x80x9cpre-heatxe2x80x9d the substrate to minimize distortion of substrate flatness during toner fusing and to reduce the temperature requirements of the second fusing. The imaged and pre-heated substrate is thereafter heated a second time using radiant or contact heating to fix the toner on the substrate.
2. Background Information
The manufacture of printing plates, including printing plates used in lithographic printing processes, using electrostatic imaging techniques is well known in the art. In such methods, the fixed toner images are the olephilic ink receptive portions of the plate, and upon contact of the plate with an appropriate ink or ink-containing solution, the desired ink image may be transferred, or xe2x80x9coffset,xe2x80x9d from the plate to an appropriate medium, such as a rubber blanket, which is then used to print onto a medium such as paper. Examples of methods of preparing printing plates which are electrostatically imaged include:
U.S. Pat. No. 3,315,600, which discloses a method for preparing a printing plate in which a support having a hydrophilic surface is provided with a covering layer, the covering layer is electrostatically imaged using a toner composition, the image is fused or fixed via heating, and the covering layer is removed from the non-imaged areas by means of an aqueous solvent. However, unlike the invention described herein, only a single heating step is employed to fix the toner image to the coated support.
U.S. Pat. No. 4,444,858, which discloses a method of preparing a lithographic printing plate in which a metal substrate is coated with a synthetic resin layer, and a toner image formed on a photosensitive sheet by an electrophotographic process is transferred and fixed to the synthetic resin layer. A solvent is used to remove the non-imaged areas of the resin layer, which are not covered by the fixed toner image. Furthermore, the toner may be removed or used as a mask. However, unlike the present invention, no second heating or fusing step is disclosed.
U.S. Pat. No. 4,457,992, which discloses an etchable electrophotographic printing plate comprising an electroconductive support coated with a light-sensitive photoconductive zinc oxide and a sensitizing dye dispersed in an organic resin binder. Such plates are typically referred to as xe2x80x9corganic photoconductorxe2x80x9d or xe2x80x9cOPCxe2x80x9d plates. The coating is applied to the substrate and dried to remove substantially all of the solvent. The resulting plate may be imaged with electrostatic toner, and the non-imaged portions of the coating are removed via a basic aqueous solution. The plate may thereafter optionally be heated to enhance plate endurance. However, unlike the invention described herein, the coating requires light-sensitive photoconductive zinc oxide to be used. In contrast, in the present invention, no light-sensitive photoconductive coating is applied to the hydrophilic surface.
U.S. Pat. No. 4,500,618 discloses an electrophotographic plate having a conductive layer thereon, which is electrically charged and imagewise exposed, followed by application of a liquid toner in a solvent. The solvent is substantially removed by heating and the material is heated a second time to fix the toner image. However, unlike the invention described herein, the coating requires light-sensitive photoconductive zinc oxide to be used. In contrast, in the present invention, no light-sensitive photoconductive coating is applied to the hydrophilic surface.
U.S. Pat. No. 6,025,100, which discloses a printing plate prepared by transferring a toner image to an image receiving element which is a support having an image receiving layer thereon. The layer contains a hydrophilic binder, TiO2 particles, and a matting agent, and the layer is cross-linked with hydrolyzed tetramethyl silicate or hydrolyzed tetraethylsilicate. However, unlike the invention described herein, there is no disclosure of a second heating or fusing of the toner to the imaged receiving element to fix the toner on the substrate.
U.S. patent application Ser. No. 09/706,521 discloses a printing plate prepared by applying an alkali soluble coating composition comprising at least one polymer composition to a hydrophilic surface on a substrate to provide the surface with at least one alkali soluble layer. The coated substrate is electrostatically imaged using a toner composition which is applied to the alkali soluble layer. The imaged substrate is heated a first time to fuse the toner composition to the alkali soluble layer, thereby protecting the underlying alkali soluble layer from subsequent contacting with developer solution in the imaged areas. The imaged plate is thereafter contacted with an aqueous alkali solution to remove undesired toner composition and the non-imaged portion of the alkali soluble layer which is unprotected by the fused toner composition, and the imaged plate is thereafter heated a second time to fix the remaining toner and underlying alkali soluble layer to the substrate. Unlike the invention described herein, a development step is required between the two thermal treatments.
Lithographic printing plates having an imageable layer overlaid upon an intermediate layer applied to a substrate are also known. For example, U.S. Pat. No. 6,014,929 discloses a lithographic plate having a rough substrate, a releasable interlayer applied to the rough substrate surface, and a radiation-sensitive layer applied to the interlayer. However, unlike the invention described herein, there is no disclosure of the use of two separate heating or fusing steps with electrostatic imaging.
However, several problems are known to be associated with the preparation of electrostatically imaged printing plates. For example, toner applied to a metal substrate often insufficiently fuses if only a standard contact fusing step is employed. This is because the metal substrate acts as a heat sink and diverts heat from the contact fuser roller, thereby resulting in insufficient energy to melt and fuse the toner. Although this problem may be avoided by using only radiant non-contact fusing, the energy required to fuse the toner using only radiant heating at the speeds typically employed in electrostatic imaging cause the metal substrate to buckle and distort due to the rapid differential expansion of the metal.
In view of the foregoing, it would be advantageous to employ electrostatic imaging of a printing plate in such a manner as to achieve adequate toner fusing and minimize or eliminate undesired buckling and distortion of the metal substrate. It is one object of this invention to provide a method of preparing an electrostatically imaged element in which adequate toner fusing is achieved and substrate buckling and distortion is avoided. It is another object of this invention to provide such an imaged element. It is yet another object of this invention to provide a method of printing using such an imaged element. The imaged element of this invention advantageously avoids rapid differential expansion of the metal substrate by controlling the rate of substrate heating. The imaged element of this invention also advantageously may be employed in high speed fusing applications which employ thick materials which require high levels of energy input. In addition, in one embodiment of this invention the first non-contact xe2x80x9cpreheatingxe2x80x9d of the substrate coupled with the second heating of the substrate using contact heating enables the contact heater rolls to squeeze the toner into the substrate surface, thereby improving toner adhesion.
A method of preparing an imaged element comprises:
(a) electrostatically imaging at least one surface of a substrate with a tone composition;
(b) heating the imaged substrate a first time using non-contact heating to a substrate preheat temperature Tp; and
(c) thereafter heating the imaged substrate a second time to substrate temperature TF, wherein the method does not comprise a development step between steps (b) and (c).
In a preferred embodiment, the substrate is an aluminum substrate. In another preferred embodiment, the substrate is coated with a polymer coating composition. The polymer composition may be solvent or aqueous soluble. The total coating weight is in the range of 0.02-5.0 g/m2, more preferably 0.2-1.0 g/m2.
In another preferred embodiment, the method further comprises a development step following step (c).
In another preferred embodiment, the method does not comprise a development step following step (c).